1. Field of the Invention
The present invention relates to an antenna device and a communication terminal apparatus including the same and particularly to an antenna device that achieves matching in a wide frequency band.
2. Description of the Related Art
In recent years, communication terminal apparatuses, such as portable phones, may require compatibility with communication systems, such as a GSM (Global System for Mobile Communication), DCS (Digital Communication System), PCS (Personal Communication Services), and UMTS (Universal Mobile Telecommunications System), as well as a GPS (Global Positioning System), a wireless LAN, Bluetooth (registered trademark), and so on. Thus, antenna devices for such communication terminal apparatuses are required to cover a wide frequency band of 800 MHz to 2.4 GHz.
The antenna devices for a wide frequency band typically have a wideband matching circuit including an LC parallel resonant circuit or an LC series resonant circuit, as disclosed in Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697. Also, known examples of the antenna devices for a wide frequency band include tunable antennas as disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065.
However, since each of the matching circuits disclosed in Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697 includes multiple resonant circuits, the insertion loss in the matching circuit is likely to increase and there are cases in which a sufficient gain is not obtained.
On the other hand, since the tunable antennas disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065 require a circuit for controlling a variable capacitance element, that is, a switching circuit for switching the frequency band, the circuit configuration is likely to be complicated. Also, since loss and distortion in the switching circuit are large, there are cases in which a sufficient gain is not obtained.